As a drive source for driving a wiper member, a wiper motor is used in a rear wiper device for wiping a rear window glass mounted on a vehicle such as an automobile. The wiper motor has an electric motor such as a brushed motor. A speed reduction mechanism and a motion conversion mechanism are contained in a gear case mounted on the electric motor. The speed reduction mechanism comprises a worm rotated by the electric motor and a worm wheel meshing with the worm. The rotation of the electric motor is reduced by the speed reduction mechanism and then transmitted to the worm wheel. A rotary motion of the worm wheel is converted into a swinging motion by the motion conversion mechanism and transmitted to an output shaft. The wiper member attached to a distal end of the output shaft is swung back and forth within a predetermined angular range. Such wiper motors are described, for example, in JP 2000-341906 and JP2006-88777.
As a wiper motor, there are a narrow-angle type (link type) with a narrow range of swing of the wiper member and a wide-angle type (differential gear) with a wide range of swing of the wiper member. The motion conversion mechanism mounted on the wiper motor of the narrow-angle type has a pivoting lever fixed to a proximal end of the output shaft, and a connecting rod converting the rotary motion of the worm wheel into the swinging motion and transmitting the swinging motion to the pivoting lever. The connecting rod is rotatably connected to the worm wheel at a position deviating radially from an axial center of the worm wheel and another end of the connecting rod is rotatable connected to the pivoting lever.
On the other hand, the motion conversion mechanism mounted on the wiper motor of the wide-angle type has a pinion gear fixed to the proximal end of the output shaft and a motion conversion member converting the rotary motion of the worm wheel into the swinging motion and transmitting the swinging motion to the pinion gear. One end of the motion conversion member is rotatably connected to the worm wheel at a position deviating radially from an axial center of the worm wheel and another end is a sector gear portion meshing with the pinion gear. A retaining plate swingably connecting the output shaft and a gear shaft set at an axial center of the sector gear portion is set up and retains meshing of the pinion gear and the sector gear portion.
Since the wiper motor of the wide-angle type has many parts due to the structure thereof, there is a problem that the degree of play in a rotational direction of the output shaft is larger than the wiper motor of the narrow-angle type. The causes of the play in the rotational direction of the output shaft include occurrence of clearance in a sliding portion of each member, backlash in meshing of the pinion gear and the sector gear portion, a deviation in meshing of the pinion gear and the sector gear portion due to play in a thrust direction of each member and the like. Particularly, in conventional wiper motors, since a contact area between a gear frame or the worm wheel and each member of the motion conversion mechanism is not designed to be fully large and each member is easily tilted, it becomes a factor that the play in the rotational direction of the output shaft increases.
For example, in the wiper motor described in JP2006-88777, since contact areas between the gear frame (gear case) and the retaining plate (swinging plate) and between the worm wheel and the motion conversion member (power conversion member) are not designed to be fully large, tilting of the motion conversion member, the retaining plate, the pinion gear (output gear) and the like cannot sufficiently be suppressed. Accordingly, when each member is tilted by reaction force by meshing between the pinion gear and the sector gear portion or the like, the meshing between the pinion gear and the sector gear deviates and the play in the rotational direction of the output shaft increases.